One of the body's simplest functions is also one of the most mysterious. Here, we take a look at scientific theories on why we yawn, and why it's so infectious.

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You’re in a meeting at work, driving,
or relaxing at home, and you just think about yawning — the next
thing you know, your mouth is stretching wide, you’re inhaling deeply, and you
don’t even really know why. Has reading this made you yawn yet?

If the answer’s yes, it’s not because
this article is boring you (we hope). Scientists know that the simple act of thinking
about yawning can trigger the part of the brain, the hypothalamus, that trips
this involuntary response.

That much we know.

But exactly why adult humans, babies,
even fetuses and most animals yawn is still a subject of much scientific debate.
According to University of Geneva researcher Adrian G. Guggisberg, MD, who wrote
a recent article about yawning in the journal Neuroscience & Biobehavioral
Reviews, yawning has been a subject of
speculation for centuries. In the fourth century B.C., Hippocrates theorized that
humans yawned to get rid of “bad air” and increase “good air” in the body
— or as we would say now, Hippocrates believed we yawn to increase
the flow of oxygen to the brain. But unfortunately for Hippocrates, Guggisberg
wrote, most scientists agree that this idea hasn’t held up under the weight of modern
research. For example, you don’t yawn more when you exercise than at any other
time, and if yawning existed to increase oxygen flow, you would.

So what really causes yawning and
makes it so infectious? Below, we rounded up a few of the top theories:

Why
We Yawn

Yawning
keeps us awake. A 2007 study at
the University of Albany in New York found that yawning
is meant to keep us awake
for a specific purpose. In this study, researchers speculated that yawns and the
contagious aspect of yawning started as an ancient social adaptation that
evolved to help groups of early humans stay alert and detect danger.

The reason for yawning proposed by the Albany
researchers is in sync with those suggested by other scientists: According to
Guggisberg’s article, theories that yawning is a social cue to indicate
drowsiness, boredom, or mild psychological stress among humans and animals are
gaining ground in the scientific community.

Yawning
indicates sexual arousal. If you
ever yawned during foreplay, you can now explain to your partner that it’s not
their fault. Several recent studies have found that a yawn could mean not
boredom, but sexual arousal. Robert
Provine, PhD, a neuroscientist, professor of psychology, and yawning expert,
explained to MSNBC that yawning is associated with a change of bodily state,
for example, from sleeping to waking or waking to sleeping. When you’re in the middle
of foreplay, a yawn could mean another state shift: from not aroused to
aroused. Provine even speculates that orgasm and yawning may share a
neurobehavioral heritage.

Yawning
cools the brain. Yawning and the
sinuses exist to cool the brain,
concluded researchers at the University of Maryland and Princeton University in
a recent study. "The brain is exquisitely sensitive to temperature changes
and therefore must be protected from overheating," the researchers wrote
in a news release. "Brains, like computers, operate best when they are
cool."

Yawning
is a symptom. If you think you
yawn more when you’re sleepy, this research could prove you’re right.
"Excessive yawning appears to be symptomatic of conditions that increase
brain and/or core temperature, such as central nervous system damage and sleep
deprivation,"
wrote Andrew Gallup, one of the study’s researchers.

Additional research suggests that excessive
yawning can be symptomatic of heart
attack or aortic dissection. This type of excessive yawning is
called a vasovagal reaction and occurs
because the body is trying to send more oxygen into the bloodstream.

Yawning
shows empathy. If you yawn when a
friend yawns, but don’t flinch when you see a stranger yawning, you might want
to thank your brain’s capacity for affection. A new Italian study connects
yawning with empathy. Researchers at the University of
Pisa found that people are more likely to yawn in response to a yawn if they
feel close to the original yawner. One of the lead researchers, Elisabetta Palagi, PhD, wrote that
the relationship between the two people was the strongest factor in whether a
yawn spread from person to person, backing up other research that has found
yawns to be a sign of empathy, not boredom.

Is
Yawning Always Contagious?

Just as there’s no clear answer to the
question of why we yawn, there’s no clear explanation for why yawns are
contagious. A University of Connecticut study found that yawning
may be a form of social bonding, which is why we yawn
when others do.

"Emotional
contagion seems to be a primal instinct that binds us together,"
Molly Helt, a doctoral student who conducted the study, told Discovery News.
"Yawning may be part of that."

According to Helt’s research, children
under age 4 appear to be immune to contagious yawning because their brains are
simply not yet sophisticated enough to pick up on the social bonding cue,
though they still experience spontaneous yawning. Similarly, researchers found
that children living with disorders on the autism spectrum were significantly
less likely to catch contagious yawns than typically developing kids because autistic
children aren’t picking on the social cues. In Helt’s study, children with the
most severe cases of autism never yawned contagiously.

Because yawning is thought to be
linked to our capacity for empathy, you might think that yawns are only
contagious for humans. Not so, say researchers at Kyoto University in Japan,
who found that chimpanzees and some baboons also experience contagious yawning.

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